1. Field of the Invention
The present invention generally relates to a switching regulator and, more specifically to a switching regulator with over-current protection.
2. Description of the Related Art
By setting various switching times of power circuits, switching regulators can provide different output voltages and currents. FIG. 1 is a block diagram illustrating a switching regulator according to the prior art. Referring to FIG. 1, a conventional switching regulator 100 comprises an error amplifier 102, a pulse width modulator 104, a gate driver 106, a tank circuit 120 and a load 130. The operation principle of the switching regulator is based on comparison of an output voltage Vout and a reference level voltage Vref for controlling the switching times of both transistor switches 108a, 108b in a switching circuit 108, thereby stabilizing the output voltage of the circuit. While the output voltage Vout is smaller than the reference level voltage Vref, the switch 108a is turned on and the switch 108b is turned off. This furnishes a path for the electrical energy stored in a commutating inductor 110 and an output capacitor 112, and thus the output voltage Vout is stepped up. Conversely, the switch 108a is turned off and the switch 108b is turned on while the output voltage Vout greater than the reference level voltage Vref. Accordingly, the commutating inductor 110 discharges and the magnetic field surrounding the coil within the output capacitor 112 starts to collapse, and thus the output Vout is stepped down.
In general, a control method is often used in the conventional switching regulator 100 by comparing an error signal VC outputted from the error amplifier 102 and a voltage level of triangular waveforms (periodic signals), directly or indirectly, so as to determine the turning-on time for each of switches 108a, 108b. That is, a duty cycle of a driving signal for controlling switches 108a, 108b is varied using a so-called pulse width modulation. In other words, the longer the turning-on time (duty cycle of the driving signal) for switch 108a and the shorter the turning-on time for switch 108b, the greater the current IL for the load 130. Contrarily, the shorter the turning-on time (duty cycle of the driving signal) for switch 108a and the longer the turning-on time for switch 108b, the smaller the current IL for the load 130. FIGS. 2A, 2B are two different block diagrams illustrating the switching regulator shown in FIG. 1 with an additional resistor. To prevent the output current IL from exceeding the limit of circuit capacity, a resistor R is added to either a source of the PMOS transistor 108a (shown in FIG. 2A) or the current path of the commutating inductor 110 (shown in FIG. 2B) in a conventional switching regulator 200 (250). The current flowing through the source of the PMOS transistor 108a is calculated by measuring the voltage over the resistor R, therefore monitoring the output current. However, there are two drawbacks for the previously discussed current measuring methods for the switching regulator with an added resistor as follows. Firstly, due to low output voltage and high current flow features, the switching regulator can not be equipped with a resistor R having a very large resistance value, or a lot of power will be dissipated, resulting in reduced efficiency of conversion. Secondly, since the current running through the source of the switch 108a is not a DC current, the output current need to be derived from peak currents.